Davis' body was found on May 13, 1988 in the 3100 block of Pepper Road west of Petaluma. She was strangled, Sonoma sheriff's Sgt. Carlos Basurto said.

Davis' mother, Mary, reported her daughter missing on May 11, 1988. Davis was last seen as she headed for a grocery store to buy diapers for one of her two children.

In May 2010, detectives with the sheriff's violent crimes unit reviewed evidence in the unsolved slaying and submitted it to the Department of Justice for DNA testing. In 2011, Chavez and Presencion were identified as potential suspects, Basurto said. The Sonoma County District Attorney's Office obtained warrants for their arrests.

Presencion is serving a 55-year prison sentence stemming from a 1989 Marin County kidnapping, rape, sodomy and burglary.

Chavez was in federal custody in Alabama in connection with immigration violations prior to his arrest in July, sources familiar with the case said.

Both men — who police say are originally from Mexico, but were living in Vallejo at the time of Davis' death — have been charged with murder and both pleaded not guilty on Aug. 16. They are scheduled to return to court on Nov. 1, where a judge could rule on a prosecution motion to obtain current DNA swabs.

"This is a case that, as far as I can tell, is going to be almost entirely based on scientific evidence, pro or con," said Dunham, the Santa Rosa attorney defending Presencion. "We deny the allegations and condemn the allegators," he said.

"The fact that there could be a match doesn't necessarily exclude any other reasonable explanation for what it was or where it was," the attorney said.

Chavez's public defender declined to discuss the case. Prosecutors also declined to comment.